The ESR test measures the sedimentation rate of aggregated erythrocytes in plasma. The rate of sedimentation is an indirect means of analyzing Rouleaux formation and aggregation of erythrocytes (red blood cells). Sedimentation rates tend to be an indicator of the presence, severity or both of some pathological conditions, such as those associated with certain disease states. Thus, the ESR test has been utilized as an indirect measure of these pathological conditions. Further, because of the simplicity of the test and materials for performing it, the ESR test has enjoyed common usage in laboratories, including popularity not only at large-scale regional clinical laboratories (e.g., serving a number of different health care facilities), within hospitals or research institutions, and even at local health care facilities (e.g., physician offices and clinics).
The history of ESR testing is relatively modern. Westergren developed the technique of performing an ESR determination as described in Alf Westergren, “Die senkungscreaktion”, Ergegn. Inn. Med. Kinderheilk., 26:577 (1924). In the Westergren method, a venipuncture blood sample mixed with an anticoagulant (e.g., including a citrate) is placed a tube. The tube is held vertically (e.g., in a rack) at room temperature, taking precautions to avoid direct sunlight, vibrations and drafts. After a time period (typically 1 hr), the distance (x) from the bottom of the resulting plasma meniscus to the top of the column of sedimenting red cells is read and the ESR value is derived (e.g., ‘ESR (Westergren 1 hr)=x mm’). The Westergren method typically employs as its measuring tube a straight tube about 30 cm long and 2.5 mm in internal diameter, thus requiring about 1 mL of blood. A citrate diluent is commonly employed as well.
Other ESR techniques are also known, such as the modified Westergren method. The Wintrobe method resembles the Westergren method, but tends to employ a shorter measuring tube (12 cm long) and omits citrate diluent from the tested blood. Historically, the Westergren and Wintrobe techniques have been manually performed. However, the techniques are now employed by semi-automated and automated instruments as well. In most of those instances, sophisticated optical detectors are employed.
As can be seen, the reliability and consistency of ESR testing can be affected by any of a number of factors, not the least of which is the nature of the tube used for containing the blood sample. For example, delays in testing or potential exposure to contaminants that deleteriously alter the surface free energy of cells are possible sources of error. It also has been recognized that variations of the nature of the measurement tube will affect test results. Specifically, it has been acknowledged that, even though plastics can be employed in certain procedures, the plastics are susceptible to (for example) plugging or plasticizer interaction with blood that are a potential for erroneous results; thus, standards prefer that blood be contained within a glass tube. See, e.g., CLSI Standard No. H2-A4, hereby expressly incorporated by reference.
Unfortunately, despite the efficacy of glass and its preference as a preferred material, many laboratory practitioners prefer the use of plastic, because of the lower risk of potential breakage and transmission of blood-borne infection.
The medical community would benefit substantially if the advantages of a glass container could be secured in a test blood tube that is also more resistant to fracture than traditional glass tubes, is capable of containing fragments if a fracture occurs, is capable of containing blood if a fracture occurs, or any combination thereof.
The use of a wrapped capillary tube for hematocrit measurement (namely the packed cell volume measurement of erythrocytes) of samples obtained from finger-stick blood draws has been proposed in U.S. Pat. Nos. 5,900,091 and 5,173,266 both expressly incorporated by reference for all purposes. In those patents a sheet of polyester film with an adhesive layer is wrapped over a small volume, thin diameter capillary tube (illustrated as having a volume of not more than 2 ml and an outside diameter of about 0.060 inches). A clay plug helps contain blood within the capillary tube. The employment of tubes of this diameter has been criticized in Clinical and Laboratory Standards Institute (“CLSI”; formerly NCCLS) Standard H2-A4, where it was also acknowledged that “[m]any so-called Westergren pipets, both glass and plastic, have an internal diameter which is less than called for in this document, i.e., less than 2.55 mm. Such pipets have been associated with spurious results, especially in specimens with a packed cell volume (PCV; “hematocrit”) greater than 0.35 (“35%”). Unfortunately, pipets adequate for all blood specimens, including those with higher PCV, are not yet widely available.” Accordingly, therefore, the selected procedure described by CLSI specifically calls for dilution of the specimen before measuring the sedimentation rate.